WASHINGTON (Reuters) - U.S. Secretary of State Hillary Clinton was sent to the hospital on Sunday with a blood clot stemming from a concussion she suffered earlier this month and was being assessed by doctors, a State Department spokesman said.

"In the course of a follow-up exam today, Secretary Clinton's doctors discovered a blood clot had formed, stemming from the concussion she sustained several weeks ago," spokesman Philippe Reines said in a statement.

"She is being treated with anti-coagulants and is at NewYork-Presbyterian Hospital so that they can monitor the medication over the next 48 hours," Reines said, adding that doctors would continue to assess her condition.

"They will determine if any further action is required," he said.

U.S. officials said on December 15 that Clinton, who earlier canceled an overseas trip because of a stomach virus, suffered a concussion after fainting due to dehydration.

They have since described her condition as improving and played down suggestions it was more serious.

Clinton's illness forced her to cancel planned testimony to Congress on December 20 in connection with a report on the deadly attack on the U.S. diplomatic post in Benghazi, Libya, in September that killed the U.S. ambassador and three other Americans and raised questions about security at far-flung posts.

Clinton, 65, has said she remains ready to testify and was expected to appear before congressional committees this month before she steps down, as planned, around the time of U.S. President Barack Obama's inauguration in late January.

Clinton, who narrowly lost the Democratic presidential nomination to Obama in 2008, is consistently rated as the most popular of his cabinet members and is often mentioned as a potential presidential candidate in 2016.